Trimercapt (referred to as "TMT-H.sub.3 " in the following description) as well as sodium salts of this trivalent acid, have been described by A.W. Hofmann - Chem. Ber. 18 (1885), 2196-2207. TMT-H.sub.3 was obtained by reacting 2,4,6-trichloro-s-triazine (cyanuric chloride) with sodium sulfide followed by acidification. A monosodium salt of trimercapto-s-triazine was also isolated.
Nakamura et al. - Japan Kokai 49/1580 (Chem. Abstr. 81, 3272 b) prepared the monosodium salt of trimercapto-s-triazine in the form of the trihydrate (TMT-Na.multidot.3 H.sub.2 O) by reacting NaHS and Na.sub.2 S with cyanuric chloride in an aqueous phase.
According to Published German Patent Specification DE-AS 22 40 549, heavy metals such as e.g. Cu, Cd, Ni, Hg, Ag, Pb can be separated as slightly soluble compounds from waste water using trimercapto-s-triazine or its water-soluble alkali-metal salts. Solid TMT-Na.multidot.3 H.sub.2 O or a 3% by weight aqueous solution of the monosodium salt of trimercapto-s-triazine (TMT-Na) is used with preference in the exemplary comments of this document. The di- and trisodium salts of trimercapto-s-triazine (referred to as "TMT-Na.sub.2 " and "TMT-Na.sub.3 " in the following description) are also suggested as possible precipitation reagents. The saturation concentration of aqueous solutions of alkali-metal-, ammonium- and alkaline-earth metal salts of mono-, di- and trimercapto-s-triazines is given as 0.01 to 25% by weight at 25.degree. C.
Published German Patent Application DE-AS 22 40 733 teaches a method of recovering heavy metal ions from ores, in which, among other things, an aqueous solution of trimercapto-s-triazine trisodium salt (TMT-Na.sub.3) is also used. No information is given about the preparation of TMT-Na.sub.2 and TMT-Na.sub.3 or their solubility in water.
Aqueous solutions of TMT-Na.sub.3 are commercially available and are used in particular for separating heavy metal from the flue gas wash water of power plants and garbage incinerators, from waste water in the mining industry and from galvanotechnical and chemical plants. The commercially available, aqueous TMT-Na.sub.3 solution has a concentration of 15% by weight (Publication TMT 15 of the Degussa company, 3/1986). The saturation concentration of TMT-Na.sub.3 in water is approximately 16% by weight at 0.degree. C.
As is apparent from Published German Patent Application DE-OS 37 29 029, TMT-Na.sub.3 was obtained for the first time as nonahydrate in crystalline form. The preparation of TMT-Na.sub.3.9 H.sub.2 O comprises the stages of reaction of cyanuric chloride with NaHS or Na.sub.2 S or with an NaHS/Na.sub.2 S mixture in aqueous medium at a pH above 7 at 20.degree.-70.degree. C., subsequent adjustment of the pH to preferably around 12.5 and cooling of the reaction mixture, at which time TMT-Na.sub.3.9 H.sub.2 O crystallizes out. After the drying, the product contains 60% by weight TMT-Na.sub.3 and 40% by weight water of crystallization.
As has already been noted above, no previous document is known which describes a disodium salt of trimercapto-s-triazine in solid form.